This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Prephenate dehydrogenase: The first committed step in tyrosine biosynthesis is the oxidative decarboxylation of prephenate to p-hydroxylphenylpyruvate (HPP). This reaction is catalyzed by prephenate dehydrogenase in the presence of NAD+ [1]. This enzyme is of paramount importance since it channels prephenate, a branch point intermediate in tyrosine and phenylalanine biosynthesis, to tyrosine synthesis. The biosynthesis of tyrosine is of critical importance for the growth and survival of enteric bacteria, yeasts, fungi and plants. Like the other aromatic amino acids, tyrosine plays dual roles in the biochemistry of the organism, acting as both a product and a precursor. In the former case, tyrosine is required for protein synthesis, whereas, in the latter example, it is a substrate for enzymes in downstream metabolic pathways. The aromatic metabolites derived from tyrosine include quinones [2, 3], cyanogenic glycosides [4], alkaloids [5, 6], flavonoids [7], and phenolic compounds derived from the phenylpropanoid pathway [7, 8]. Since many of these compounds are involved in primary biological processes, they are essential for viability. In plants, for example, flavonoids are important for normal development as they are involved in auxin transport [9-11], pollen germination [9, 12, 13], and signaling to symbiotic microorganisms [9, 14]. We have recently determined the crystal structure of prephenate dehydrogenase however we propose that our understanding of the mechanism of this enzyme hinges on determining its crystal structure in complex with know ligands. We recently obtained crystals of prephenate dehydrogenase in co-crystallization studies using prephenate, (4-hydroxyphenyl)pyruvate and tyrosine separately. We are requesting beam time for x-ray data collection to determine the structure of prephenate dehydrogenase in complex with these ligands. Shikimate dehydrogenase: The shikimate pathway is involved in the biosynthesis of the aromatic amino acids, folates, vitamins, quinones and a variety of other aromatic compounds in bacteria, plants, fungi and apicomplexa parasites. Some of the aromatic compounds are essential for the survival of these organisms;as a result, the shikimate pathway has been an attractive target for the design of antimicrobial and herbicidal agents. This bifunctional enzyme dehydroquinate dehydratase-shikimate dehydrogenase (DHQ-SDH) catalyzes the dehydration of dehydroquinate to dehydroshikimate followed by the reduction of dehydroshikimate to shikimate in the shikimate pathway. We have recently determined the crystal structure of DHQ-SDH. We have recently made a number of active site mutants, which we proposed are important for our understanding of the catalytic mechanism of this enzyme. We have now obtained crystals of these mutant variants and are requesting beamtime for x-ray data collection. MarR: The biological route by which an organism acquires antibiotic resistance can stem from one or more mechanisms, most of which are not well understood. One such mechanism is multiple antibiotic resistance (MAR) in which microbes reduce their intracellular concentration of antibiotics by upregulating the expression of drug efflux pumps, which actively remove drug-like compounds from the cell . We have recently determined the crystal structure of a multiple antibiotic resistance repressor protein (marR) and have conducted co-crystallization studies with a number of drug compounds. We have obtained crystals of marR in a number of co-crystallization studies and are requesting beamtime for x-ray data collection. Our goal is to determine the structure of marR in complex with the different drug compounds, which is important for our understanding of the mechanim of multiple antibiotic resistance.